ABSTRACT A woman diagnosed today with an ovarian cancer has only a minimally improved chance of long term survival compared to a woman diagnosed 40 years ago. Two different yet related observations may provide an opportunity to improve the outlook of ovarian cancer patients. Both involve DNA damage and repair. 1) The most effective and widely used drugs, the platinum compounds (cisplatin and carboplatin), lose their effectiveness over time ? drug resistance develops. An important (but not the only) reason for drug resistance is an acquired ability of ovarian cancers to repair the damage caused by cisplatin DNA adducts. 2) A different class of drugs that induce DNA damage, the PARP inhibitors, has recently been approved by the FDA for patients with DNA repair defects, such as germline or acquired mutations in BRACA1/2. Unfortunately, patients with defects in DNA repair genes represent only a small fraction of ovarian cancer patients. We propose that targeting the mitochondrial protein sideroflexin 4 (SFXN4) may provide a path to addressing both these problems. We discovered that reducing the levels of SFXN4 disrupted Fe-S cluster formation in the mitochondria and cytosol of ovarian cancer cells. We propose that the loss of Fe-S clusters will have major cellular consequences affecting ovarian cancer. First, reduction of Fe-S proteins will increase oxidative stress and DNA damage via the acquisition and redistribution of cellular iron. Second, because critical enzymes involved in NER (nucleotide excision repair) and HRR (homologous recombination repair) require Fe-S clusters for their function and stability, targeting SFXN4 will reduce the activity of DNA these repair pathways. Given the possibility that reduction of SFXN4 could both induce DNA damage and inhibit DNA repair, we further hypothesize that disruption of SFXN4 might enhance the efficacy of platinum-based compounds and expand the effectiveness of PARP inhibitors to ovarian cancers without defects in DNA repair: i.e., render these cancers exquisitely sensitive, like BRCA-mutant cells, to platinum-based drugs and PARP inhibitors. In pilot experiments, we indeed observed that inhibiting SFXN4 1) inhibited DNA repair proteins; 2) enhanced sensitivity to cisplatin; 3) sensitized ovarian cancer cells to PARP inhibitors. We propose three Specific Aims to study the effects of SFXN4 in ovarian cancer. In Aim 1, we explore how and to what extent reduction of SFXN4 increases oxidative stress, limits DNA repair and enhances DNA damage. In Aim 2, we examine the role of targeting SFXN4 in enhancing the effectiveness of both platinum- based compounds and PARP inhibitors in cell cultures and mouse models. In Aim 3 we determine the precise molecular site and mechanism by which SFXN4 functions in Fe-S cluster biogenesis, and directly link this effect to the reduction of DNA repair proteins in NER and HRR pathways. Impact: targeting SFXN4 may represent an opportunity to enhance the efficacy of platinum-based drugs and extend the use of PARP inhibitors to the majority of ovarian cancer patients with normal DNA repair function.